Multilayer ceramic structures with internal conducting layers of metal are well known in the prior art. In general such structures are formed from ceramic green sheets prepared from suspensions of ceramic powders dispersed in thermoplastic polymer and solvent. Conductors are deposited on some of the green sheets in a pattern, usually by screen-printing a paste consisting of a metal powder, an organic binder and solvent. The sheets with conductors on them may also have via or feed-through holes punched in them, as may be required for interconnections between layers in the final multilayer structure. The green sheets are stacked with appropriate registration of the various levels and laminated to form a multilayer structure. This structure is then fired to drive off the organic binders and to sinter the ceramic and metal particulates. The multilayer structure may be diced into smaller units before firing.
Base-metal conductors have been used in some multilayer ceramic structures. In such cases the structures are fired in an atmosphere with low oxygen content to protect the conductors from being oxidized during the firing process. For example, Ni electrodes have been used in multilayer ceramic capacitors containing titanate or zirconate dielectrics, and Mo-Mn alloy has been used for multilayer substrates based on aluminum oxide. These base metals do not melt at the high firing temperatures needed to sinter the ceramic (&gt;1350 C.). Attempts have been made to use highly conducting copper electrodes in multilayer structures but, because copper has a low melting point (1083 C.), ceramic compositions must be modified by adding sintering aids such as low-melting glasses or fluxes to achieve a dense hermetic structure when fired below the melting point of copper.
A major difficulty with using copper electrodes in multilayer ceramic structures is the removal of organic binders before the ceramic sinters. This can lead to discoloration due to carbon entrapment, incomplete densification of the ceramic, or nonuniform or delaminated electrodes. A method to avoid the problem is claimed in U.S. Pat. No. 4,551,357 in which an organic binder is used in the electrode paste which decomposes at a temperature higher than the thermoplastic polymer in the green ceramic sheet. Firing is carried out in a nitrogen atmosphere with not more than 200 ppm oxygen, or in a gas mixture of nitrogen, hydrogen and water vapor. Alternatively, a method for processing multilayer ceramic substrates with copper electrodes is claimed in U.S. Pat. No. 4,234,367 in which organic residues are removed by using long (&gt;12 hours) presintering heat treatments in an atmosphere of nitrogen followed by a mixture of water vapor and hydrogen. In addition, U.S. Pat. No. 4,308,570 and U.S. Pat. No. 4,101,952 disclose multilayer structures in which a pair of copper electrode layers are separated by a dielectric of thickness of about 80 microns in a multilayer ceramic capacitor. Ethyl cellulose was used in the electrode paste and the multilayer structures were sintered in a mixture of carbon dioxide and carbon monoxide.
It has been found that whereas this latter process works well for multilayer structures with few layers of copper conductor, the difficulty of removal of the organic binder from the electrodes in high layer count parts (e.g., five or more) results in inadequate control over the sintering of the copper to achieve continuous layers of uniform thickness.